Gas furnace Venting - Current situation is Electricity is cheaper then Gas!!!

I have an older house ( 1940's ) that is 'partially' insulated with R12. I am located on the Rainy West coast, just north of Seattle. There is no way that this place will be R40! lol!

Energy Costs Electricity is measured in kWh's and Natural gas is measured in Giga Joules. Both are measures of work, and we can figure out how many kWh's make up a Giga Joule. Once I did this it's easy to lookup what I am paying for electricity vs what I am paying for Gas.

Cost Comparison: (approx from memory)1 GJ of electricity is about $16.00 cdn1 GJ of gas is about $12.00 Cdn.Gas here only a 20% cheaper than electricity.

Once we factor in that the Lennox G20 furnace is 78% efficient, the cost comparison for burning Natural Gas vs electricity makes them both 'identical'!

But this is under 'ideal' conditions.. My furnace is not installed under the most ideal conditions.

The Lennox G20 burns room air and sends it up the chimney. As a result, the installer ran a pipe from the furnaces cold air return, to the outside of the house to ensure that there is an air source for combustion.

So everytime the furnaces fan is active, we are pulling in lots of cold outside air (burner on or not).

All these venting factors make Natural GAS heating *MORE* expensive than electricity! I got this idea from another home owner in my area that dumped gas for electrically heated oil radiators in each room. Their heating costs have dropped.

Question - Would it not be smarter to build a sheet metal box that covers over the Lennox's burner and flue intakes, and connect this to the outside air? Is this safe to do, what are the pitfalls if any?

My house is a 1940's wartime house with entire walls without any insulation.